IP-based mobile system includes at least one Mobile Node in a wireless communication system. The term “Mobile Node” includes a mobile communication unit, and, in addition to the Mobile Node, the communication system has a home network and a foreign network. The Mobile Node may change its point of attachment to the Internet through these other networks, but the Mobile Node will always be associated with a single home network for IP addressing purposes. The home network has a Home Agent and the foreign network has a Foreign Agent—both of which control the routing of information packets into and out of their network.
The Mobile Node, Home Agent and Foreign Agent may be called other names depending on the nomenclature used on any particular network configuration or communication system. For instance, a “Mobile Node” encompasses PC's having cabled (e.g., telephone line (“twisted pair”), Ethernet cable, optical cable, and so on) connectivity to the wireless network, as well as wireless connectivity directly to the cellular network, as can be experienced by various makes and models of mobile terminals (“cell phones”) having various features and functionality, such as Internet access, e-mail, messaging services, and the like. And, a home agent may be referred to as a Home Agent, Home Mobility Manager, Home Location Register, and a foreign agent may be referred to as a Foreign Agent, Serving Mobility Manager, Visited Location Register, and Visiting Serving Entity. The terms Mobile Node, Home Agent and Foreign Agent are not meant to be restrictively defined, but could include other mobile communication units or supervisory routing devices located on the home or foreign networks. Foreign networks can also be called serving networks.
Registering the Mobile Node
Foreign Agents and Home Agents periodically broadcast an agent advertisement to all nodes on the local network associated with that agent. An agent advertisement is a message from the agent on a network that may be issued under the Mobile IP protocol (RFC 2002) or any other type of communications protocol. This advertisement should include information that is required to uniquely identify a mobility agent (e.g. a Home Agent, a Foreign Agent, etc.) to a mobile node. Mobile Nodes examine the agent advertisement and determine whether they are connected to the home network or a foreign network.
If the Mobile Node is located on its home network, information packets will be routed to the Mobile Node according to the standard addressing and routing scheme. If the Mobile Node is visiting a foreign network, however, the Mobile Node obtains appropriate information from the agent advertisement, and transmits a registration request message to its Home Agent through the Foreign Agent. The registration request message will include a care-of address for the Mobile Node. A registration reply message may be sent to the Mobile Node by the Home Agent to confirm that the registration process has been successfully completed.
The Mobile Node keeps the Home Agent informed as to its current location by registering a “care-of address” with the Home Agent. The registered care-of address identifies the foreign network where the Mobile Node is located, and the Home Agent uses this registered care-of address to forward information packets to the foreign network for subsequent transfer onto the Mobile Node. If the Home Agent receives an information packet addressed to the Mobile Node while the Mobile Node is located on a foreign network, the Home Agent will transmit the information packet to the Mobile Node's current location on the foreign network using the applicable care-of address.
Foreign Agent Incoming and Outbound Traffic
The Foreign Agent participates in informing the Home Agent of the Mobile Node's current care-of address. The Foreign Agent also receives the “incoming” information packets addressed to the Mobile Node after the information packets have been forwarded by the Home Agent. Further, the Foreign Agent serves as a router for “outbound” information packets generated by the Mobile Node while connected to the foreign network depending on the delivery style chosen.
Under RFC 3024, after a Mobile Node arrives at a foreign network, it listens for agent advertisements and selects a Foreign Agent that supports its desired communications. The Mobile Node registers through the selected Foreign Agent. At this point, and depending on how the Mobile Node wishes to deliver packets to the Foreign Agent, the Mobile Node may also request either the Direct or the Encapsulating Delivery Style.
In the Direct Delivery Style, the Mobile Node designates the Foreign Agent as its default router and proceeds to send packets directly to the Foreign Agent, that is, without encapsulation. The Foreign Agent intercepts those packets, and reverse tunnels them to the Home Agent. In the Encapsulating Delivery Style, the Mobile Node encapsulates all its outgoing packets to the Foreign Agent. The Foreign Agent decapsulates and reverse tunnels those packets to the Home Agent, using the Foreign Agent's care-of address as the entry-point of this new tunnel.
Unicast, Broadcast and Multicast Messages
Unicast is the term used to describe communication where a piece of information is sent from one point to another point. In that situation, there is just one sender, and one receiver. A unicast transmission, in which a packet is sent from a single source to a specified destination, is the predominant form of transmission on the Internet.
Mobile Nodes sometimes transmit broadcast and multicast messages from their location on a foreign network. Broadcast is the term used to describe communication where a piece of information is sent from one point to all other points on another network, such as a foreign or home network. In this case there is just one sender, but the same information is sent to all connected receivers on that network.
Multicast is the term used to describe communication where a piece of information is sent from one or more points to a set of other points. In this case there may be one or more senders, and the information is distributed to a set of receivers. Multicasting is the networking technique of delivering the same packet simultaneously to a group of clients. Unlike broadcast transmission, however, multicast clients receive packets only if they have previously elect to do so by joining the specific multicast group address. Membership of a group is dynamic and controlled by the receivers. The Foreign Agent can recognize the multicast or broadcast address, and the Foreign Agent can distinguish those addresses from unicast addresses.
Encapsulation Delivery Style
When the Mobile Node have their unicast or broadcast/multicast (“BC/MC”) packets reverse-tunneled by the Foreign Agent back to the Home Agent, the Mobile Node must use the encapsulating delivery style under RFC 3024. The encapsulation delivery style requires that the Mobile Node place an additional header on each outbound packet sent from the Mobile Node to the Foreign Agent. This encapsulation delivery style delivers the datagram only to the Foreign Agent, and the Foreign Agent decapsulates it and then processes it as any other packet from the Mobile Node, namely, by reverse tunneling it to the Home Agent.
Every time a Foreign Agent operating under RFC 3024 receives an encapsulated packet from a Mobile Node, the Foreign Agent will assume that reverse tunneling has been chosen and that the packet (regardless of whether unicast, multicast or broadcast) needs to be sent to the Home Agent without consideration of the type of datagram. With that assumption, all the encapsulated outbound traffic received at the Foreign Agent from the Mobile Node will be decapsulated and processed by the Foreign Agent to reverse tunnel it to the Home Agent.
This encapsulation of outbound BC/MC packets places an additional overhead demand on the Mobile Node that may not be necessary in all circumstances, and the encapsulation delivery style requires the Foreign Agent to perform the decapsulation/encapsulation actions in all situations where it receives an encapsulated packet, which may not be necessary all the time. It would be beneficial to avoid incurring these overhead losses for certain BC/MC packets, which would be supported by selectively negotiating the delivery style for certain BC/MC packets.
After the Foreign Agent transmits an encapsulated BC/MC packet back to the Home Agent with reverse tunneling, any responses to the BC/MC packet addressed to the Mobile Node must be transmitted through the Home Agent and tunneled through the home network before being transmitted to the Foreign Agent and onto the Mobile Node. This additional step of transmitting all responses through the Home Agent in all circumstances is required because of the reverse tunneling conducted by the Foreign Agent, but responding in that manner may constitute an unnecessary overhead loss that the system may want to avoid. It would be beneficial to have a choice of obtaining a more direct response to the Foreign Agent for certain BC/MC packets, which would be supported by selectively negotiating the delivery style for certain BC/MC packets.